1. Field of the Invention
The present invention relates to a solid-state image pickup device used in a video camera or the like and a driving method therefor.
2. Description of the Related Art
Noise superimposed on pixel signals of solid-state image pickup devices used in video cameras or the like can be broadly divided into random noise occurring in temporally and spatially random states, and fixed pattern noise occurring at the same position on an output image as vertical stripes or horizontal stripes. Random noise appears as uneven noise as if an entire screen is being viewed through frosted glass. Random noise occurs due to optical shot noise or thermal noise, and markedly varies with ambient temperature. On the other hand, fixed pattern noise occurs due to a variation of electrical characteristics between a pixel circuit and a reading circuit in a solid-state image pickup device. In particular, if different types of fixed pattern noises occur in different pixel columns, vertical stripes appear on an output image. These vertical stripes are conspicuous to human eyes.
In order to remove such different types of fixed pattern noises, the following technique is disclosed. First, a fixed pattern noise component is detected from signals (hereinafter referred to as noise measurement signals) output from a dummy line that is not connected to pixels capable of performing photoelectric conversion and is then stored. Subsequently, when a pixel signal (hereinafter referred to as an effective pixel signal) is output at the time of image capturing, the detected fixed pattern noise component is subtracted from the effective pixel signal so as to cancel the noise component.
Furthermore, a technique is disclosed for detecting a fixed pattern noise component reflecting variations in electrical characteristics among reading transistors in a row direction without being affected by a dark current (for example, see, Japanese Patent Laid-Open No. 2005-176061). Japanese Patent Laid-Open No. 2005-176061 discloses a technique for accurately canceling a fixed pattern noise component using non-effective pixels as a dummy line. Each of the non-effective pixels includes an amplification transistor and a reset transistor, but includes no photodiode.
In the technique disclosed in Japanese Patent Laid-Open No. 2005-176061, the number of dummy lines M for outputting a noise measurement signal is equal to or greater than the square of M, and the averaging of noise measurement signals output from each pixel column is performed so as to reduce the amount of a random noise component superimposed on a noise measurement signal to one Mth of the total amount of noise component or less. If the amount of the random noise component superimposed on a noise measurement signal is equal to or less than a quarter of the total amount of the noise component, random noise cannot be seen on a displayed image. By averaging noise measurement signals of a plurality of frames, a similar effect can be obtained with a smaller number of dummy lines. Furthermore, a solid-state image pickup device having a plurality of driving modes is known.
In Japanese Patent Laid-Open No. 2005-176061, there is no description about noise measurement signal acquisition timing after a driving mode has been changed. In order to meet a current request for noise reduction in a solid-state image pickup device, it is required to perform averaging of a larger number of noise measurement signals. In the above-described technique, although averaging of noise measurement signals output from M2 (M=4) dummy lines, that is, sixteen dummy lines, is performed, averaging of a larger number of noise signals is required.